my bike has never fallen off
works fine. mine has a sleeve to fit both hitch receivers though. bikes stay on the rack and the securing strap deals are pretty nice (and replaceable). bikes bounce around and swing a lot but i have never seen a hitch mount that doesnt - and yes, i use the zip stick.
i prefer the bikes on the roof or in the back of a truck. i guess if youre a baller and drive a hummer or escalade or similar, a roof rack wont work for you so this is a good option.